Replace deprecated io/ioutil from golang

[seokho-son]

Replace deprecated functions. (Deprecation of io/ioutil as of Go 1.16)

https://go.dev/doc/go1.16#ioutil

Deprecation of io/ioutil
The [io/ioutil](https://go.dev/pkg/io/ioutil/) package has turned out to be a poorly defined and hard to understand collection of things. All functionality provided by the package has been moved to other packages. The io/ioutil package remains and will continue to work as before, but we encourage new code to use the new definitions in the [io](https://go.dev/pkg/io/) and [os](https://go.dev/pkg/os/) packages. Here is a list of the new locations of the names exported by io/ioutil:

[Discard](https://go.dev/pkg/io/ioutil/#Discard) => [io.Discard](https://go.dev/pkg/io/#Discard)
[NopCloser](https://go.dev/pkg/io/ioutil/#NopCloser) => [io.NopCloser](https://go.dev/pkg/io/#NopCloser)
[ReadAll](https://go.dev/pkg/io/ioutil/#ReadAll) => [io.ReadAll](https://go.dev/pkg/io/#ReadAll)
[ReadDir](https://go.dev/pkg/io/ioutil/#ReadDir) => [os.ReadDir](https://go.dev/pkg/os/#ReadDir) (note: returns a slice of [os.DirEntry](https://go.dev/pkg/os/#DirEntry) rather than a slice of [fs.FileInfo](https://go.dev/pkg/io/fs/#FileInfo))
[ReadFile](https://go.dev/pkg/io/ioutil/#ReadFile) => [os.ReadFile](https://go.dev/pkg/os/#ReadFile)
[TempDir](https://go.dev/pkg/io/ioutil/#TempDir) => [os.MkdirTemp](https://go.dev/pkg/os/#MkdirTemp)
[TempFile](https://go.dev/pkg/io/ioutil/#TempFile) => [os.CreateTemp](https://go.dev/pkg/os/#CreateTemp)
[WriteFile](https://go.dev/pkg/io/ioutil/#WriteFile) => [os.WriteFile](https://go.dev/pkg/os/#WriteFile)

[seokho-son]

/approve